Control system for ovens



M. MILES 3,251,980

CONTROL SYSTEM FOR OVENS May 17, 1966 Original Filed Nov. 21, 1962Des/rec Mea/ f m 0 (6/ Mea/ v emp. fem/Q aj kfea/ AW /z) v United StatesPatent 4 Claims. (Cl. 219-516) This invention relates to control systemsfor ovens, and more particularly to control systems for roasting meatsor other food products in which it is desirable to maintain control-ofthe internal temperature of the food product being cooked.

This application is a division of the copending application of MarshallMiles, Serial No. 239,112, filed November 21, 1962, and subsequentlyabandoned.

Presently accepted methods of roasting meat involve cookingin an ovenwhich is held at an approximately constant temperature, usually 300 F.to 350 F. and allowing meat to cook until its internal temperaturereaches a point indicating the desired degree of doneness. This pointis, for example, 160 F. for medium beef. The length of time required tocomplete the roast is thus a function of the weight and quality of themeat as well as oven temperature. These variables are complexlyinterrelated and the cook has no means available to control conditionsso that the roast will be completed to the desired internal temperatureat the time of serving. The only choice with present systems is to use ahigh enough oven temperature so that completion is certain beforeserving time and to try to keep the roast warm it it should be completedprior to serving time. This frequently means the cook must start theroasting process at an inconvenient time and/ or serve a roast that hasbeen dried by an extended delay before serving.

It is well established that low temperature long time roasting of meatsresults in improved flavor and tenderness, as well as less shrinkage andfuel consumption. It is, therefore, desirable to provide a system inwhich the oven temperature is programmed as a function of the meattemperature. In such a system the temperature of the oven is controlledin accordance with the temperature of the meat so that when the meatapproaches the desired temperature representative of the degree ofdoneness, the oven temperature is at a level just sufficient to maintainthe meat at that temperature without further cooking.

It is, therefore, a primary object of this invention to provide acontrol system for cooking meat or the like in which the cookingtemperature is programmed in accordance with the internal temperature ofthe meat so as to control the maximum internal temperature thereof.

In one embodiment of this invention the teachings thereof are used in akeep warm system in which the meat is brought up to temperature over anuncontrolled time period and maintained at the desired temperature untiltime for serving. Presently available systems of this type simply turnthe oven temperature down at a preset time and/ or temperature. Theproblem with this method is that considerable cooking occurs during theoven coast down. In order to prevent this from becoming a major factor,some manufacturers have proposed the use of a fan to get rid of unwantedoven heat. Others have anticipated the coast down by having a clockcause the oven to .decrease in temperature prior to the meat actuallyreaching the desired temperature.

Briefly, the keep warm system embodying the present invention comprisesa settable thermostat which is operable responsive to oven temperaturefor controlling the heat therein. The thermostat is arranged andconstructed so as to be controllable by a second heating source which isoperable responsive to the internal temperature of the meat beingcooked. The auxiliary heating source comprises a heater winding adjacentthe bimetal element of the thermostat which is connected in an electriccircuit including a temperature responsive resistance probe insertablein the meat. Thus, as the temperature of the meat increases, theauxiliary heat source-causes the bimetal element in the thermostat toreact so as to reduce the oven temperature. When the temperature of themeat reaches its desired level, the oven is controlled so that itsaverage temperature is just enough to maintain the meat at the desiredlevel. It is, therefore, an

object of this invention to provide an oven control systern in which thetemperature of the oven is controlled in accordance with the internaltemperature of the meat so as to bring the internal temperature to adesired temperature level and maintained thereat.

Probably, the most desirable method of roasting would use the maximumtime available for cooking by adjusting the oven temperature throughoutthe cooking process so that the meat temperature rises uniformly andjust reaches the desired temperature indicative of the degree ofdoneness at the moment of serving time. There is herein described andclaimed a second embodiment of the invention in which the oventemperature is controlled not only by the meat being cooked but also asa function of time so that it will reach its desired temperature at theend of the preselected time. Briefly, this system comprises an oventhermostat with an auxiliary heat source controlled by a temperatureresponsive resistance probe for the meat. In addition, however, there isprovided a timing means which causes the auxiliary heat source to varyalso as a function of time. Thus the'oven thermostat varies the oventemperature as a function of both the internal temperature of the meatand time.

This invention, as'hereinbefore briefly described, will be betterunderstood upon further reading of this specification especially takenin view of the accompanying drawings in which:

FIG. 1 is a schematic diagram of a first embodiment of the invention inwhich the control system operates as a keep warm system to control thetemperature of the oven as a function of the internal temperature of themeat being cooked so as to bring the temperature of the meat to adesired temperature level based on the desired degree of doneness andmaintained at that temperature until serving;

FIG. 2 is a schematic diagram of a second embodiment of this inventionwherein the oven temperature is controlled as a function of both theinternal temperature of the meat and time so that the internaltemperature of the meat is slowly brought up to the desired internaltemperature level over a desired time interval to reach the desiredtemperature at the end of the desired time interval; and

FIG. 3 is a graphic representation of the oven and meat temperatures asa function of time for the embodiments shown in FIGS. 1 and 2.

Referring now to the keep warm control system of FIG. 1 there is shown athermostat unit 10 which is positioned in the oven to be subjected tothe atmosphere therein and a meat thermometer unit 12 to be locatedelsewhere as hereinafter described. In this system the oven heaterelement 14 is connected in series through normally closed contacts 16and 18 as well as the bimetal strip 20 to a line source voltage at 22.Closed contacts 16 and 18 allow current to flow through the heaterelement 14 causing the oven to heat. Contact 18 is supported on thebimetal strip 20 and the tension with which it causes contact 18 to meetwith contact 16 is adjustable by means of the cam and lever arrangement26 or any other suitable means. This provides a manual means for settingthe desired temperature;

The bimetal 20 is exposed to oven temperature and is so arranged thatincreasing temperatures will cause contact 18 to separate fromstationary con-tact 16. Thus, at a temperature established by tensionfrom the adjusting means 26 the contacts will separate until the oventemperature drops sufficiently to cause them to remake. Accordingly, theaverage oven temperature will be maintained at a set point establishedby adjustment providing that there is no other agency affecting thetension between the contacts 16 and 18.

However, the bimetal 26 has a heater winding 28 surrounding or closelyadjacent there-to which may be subjected to an electric current ashereinafter described. The net effect of this part of the thermostaticunit is such that, although a set temperature may have been establishedby appropriately adjusting the tension of the bimetal 29, the set pointcan be altered by energizing the heater Winding 28. The direction ofaltering is such that increasing 7 the temperature of heater 28 willchange the set point to'a lower temperature. In other words, the oventemperature can be regulated by the amount of energy put into heater 28;the more current flowing through 28 the lower the established oventemperature. This is because additional heat from the heater 28 willcause the bimetal ZiL and hence contact 18, to deflect further to theleft as viewed in FIG. 1 requiring the oven temperature to drop to alower value during its off time to remake the contacts 16, 18.

In the meat thermometer unit 12 is a temperature indithereabout. Thetemperature setting arm 34 is manipu-' latable for setting the desiredtemperature of the food in order to energize an alarm. The indicatingarm 36, however, is operable by the electrical circuit components forrotation responsive to the temperature of the meat which is sensed by athermistor, 39 located in a probe insertable into the meat.

The indicator arm 36 is rotated about its pivot 38 by means of adeflectable bimetal element 40 which is 0perated responsive to heatgenerated in its heater winding 42 wrapped'therearound. The bimetal 40is a U-shaped element having one of its legs in rigid support at its end44. The other leg 45 of the element 40 has the heater winding 42 wrappedtherearound and carries a pin 46 at its extreme end which operationallyengages a slot 48 in the pivotally supported end 50 of the indicator arm36. Thus, as current flows through the winding 42, through a circuit tobe hereinafter described, the heat generated causes the arm 45 todeflect concavely to the right as viewed in FIG. 1 so that the pin 46interacts with the slot 48 on .the arm 36 to cause it to rotate in aclockwise direction. The bimetal element 40 is U-shaped to providevariable ambient temperature compensation in a well known manner. 7

One end of the heater winding 42 is connected to jack 52 which also isconnected to the thermistor 39. The other end of the winding 42 isconnected by lead 53 to a second bimetal element 54 which forms a partof a voltage regulator 55 in the circuit for energizing the winding 42.The bimetal 54 carries a contact 56 which is engageable with apositionally fixed contact 58. The contact 58 is electrically connectedto jack 59 which also receives lead 60 from one end of the heaterwinding 28 in the thermostat unit 10. The other end of the heaterwinding 28 is connected by lead 62 to the secondary 64 of a step-downtransformer 66 supplying reduced voltage thereto from a standard linesource.

The regulator 55 is included in the thermometer unit 12 for producing arelatively constant mean effective vol-tage to energize the heatingwinding 28 and the indicator ar-m bimetal winding 42. The bimetalelement 54 in regulator 55 is also U-shaped having the end of one legrigidly supported with the other leg supporting the contact 56. This legcarries a heater winding 7 0 having one end connected to ground and theother end connected to the contact56 so that the winding is energizableby the secondary of the transformer 66 through the circuit includingground, winding 70, contacts 56 and 58, line 60 and heater winding 28.Since the winding 70 is energized through the contacts 56, 58, thecontacts are caused to intermittently make and break cylically inaccordance with a period determined by the spring tension between thecontacts. The regulator operates to produce a relatively constant meaneffective voltage as follows: If the line voltage at the transformer 66increases, the period of the cycle operation of the regulator 55 alsoincreases, thus reducing the mean effective voltage applied to thewinding 62. Conversely, when the line voltage decreases, the period ofcyclic operation of the regulator also decreases which causes anincrease in the mean effective voltage applied to the heater winding 42.The U-shape of the bimetal element 64 again provides ambient temperaturecompensation for the voltage regulator 55.

The average current fiowing into meat thermometer 12 through the heaterwinding 28 will be dependent on the resistance 'of the thermistor 39which is insertable into the meat being cooked. This current will resultin power developed within heater 28 and will vary as a function of theresistance of the thermistor. If the thermistor is buried within a pieceof meat which is cold the amount of power dissipated in the heater willbe small because of the negative temperature coefiicient of thermistors.Thus, there will be comparatively little deflection of bimetal 20 andits contact '18 in the thermostat unit 10 so that the operation of thethermostat unit will be dependent primarily upon the temperature in theoven. However, as the temperature of the meat increases,.the resistanceof the thermistor 39 decreases and the power dissipated in the heaterelement 28 increases causing a greater deflection of contact 18 awayfrom its stationary mate 16. As this deflection increases the set pointwill gradually decrease resulting in a gradual lowering of the oventemperature. Further increase of meat temperature will result in furtherdecrease of oven temperature until an equilibrium state occurs.

Referring now to the temperature; vls. time graph of FIGS, curve (a)represents the programmed oven temperature for cooking the meat to adesired temperature. Curve (b) represents the internal temperature ofthe meat as it approaches the desired temperature.

Thus at the beginning of the cooking time, when the meat is still cool,the oven temperature is at its maximum temperature. As the meattemperature approaches the desired temperature T, the oven temperatureis gradually decreased so that at the time 2. reaches the desired valuethe oven temperature is only slightly greater to maintain the meattemperature thereat and supply any heat losses in the system.

It is to be noted in the system of FIG. -1 that the thermostat unit "10and the thermometer unit 12 may be used in a conventional manner to cookfoodstuffs in the oven merely by operating a switch which removes theheater winding 28 on the circuit and replaces it with resistor 77. Thethermostatic unit 16 is therefore dependent completely on the oventemperature and the variations in the resistance of the thermistor serveonly to operate the indicator arm '34 of the meter. The contacts 79 and81 on the indicator arm 84 and settable arm 36, respectively, may beused to complete a circuit to an alarm device if desired to indicate theattainment of a desired temperature.

Reference is now made to FIG. 2 wherein is shown a system forcontrolling the oven temperature as a function of time, as well as afunction of the internal temperature of the meat.

The oven thermostat unit 410 shown in FIG. 2 is somewhat different thanthat described for the embodiment of FIG. 1, but it operates insubstantially the same manner and is in fact interchangeable therewithso that either thermostat unit or 10' will work with either the systemsof *FIG. 1 or FIG. 2. In the device 10' of FIG. 2 the oven heaterelement :14 is connected in series through normally closed contacts '16,'18 with the line source voltage; Closure of contacts 16, 18' allowscurrent flow through the heater element '14 causing the oven to heat.Contact 18 is supported on bimetal strip 78 whose tension is adjustableby cam and lever means 26' in a manner similar to the unit 10 of FIG. 1.The bimetal 78 is exposed to oven temperatures and is so arranged thatincreasing temperatures will cause contact 18 to separate from contact116' mounted on a separate bimetal 80. The auxiliary heater winding 28surrounds the ibimetal 80 providing the control of the thermostat unitL10 from the auxiliary means as hereinafter described.

The meat thermometer unit 1 2 of FIG. 2 is shown in its simplest formincluding thermistor 39 and the stepdown transformer 66 in seriesconnection with the auxiliary heater winding 28'. The voltage regulatorand temperature indicator units may be included in the unit 12' but areeliminated in this description for purposes of simplicity.

Also included in the series circuit is a variable resistor 82 which maybe adjusted to operate through a desired portion of its range by timermeans 84 hereinafter described. Thus, for a given setting of thevariable resistor 82 the thermistor 39 will cause the set point of theoven thermostat unit 10' to lower with increasing temperatures of themeat so that the oven temperature will be reduced therewith and the ovenand meat temperature curves would be similar to curves (a) and (b) shownin FIG. 3. .However, it is the purpose of this system to control thetemperatures so that the meat just reaches the desired temperature in apreselected time t. The flavor and tenderness of the meat is improvedwith less shrinkage and fuel consumption with the low temperature, longtime roasting of the meats.

The timer means 84 comprises a cam 86 with a foliower 88 operable toactuate the variable resistor 82. The cam 86 is driven by a motor 90'which is operable through gear means (not shown) to rotate the camthrough a full cycle in an extended length of time such as 12 hours. Thecam 86 is presettable from its at rest position as shown in FIG. 2 in acounterclockwise direction at any point representative of a desired timeinterval up to 12 hours. An indicator arm 90 is 'connected to therotatable cam 86 to indicate on a dial face the desired time setting.The cam 86 is settable from a maximum resistance setting of resistor 82at zero time to a minimum resistance setting for a desired 12-hourcooking period. Thus, the resistance in series with the thermistor S9and heater element 28' will increase as a function of time so that lesspower is delivered to the heater 28 causing the temperature of the ovento increase with the passage of time.

The purpose of the thermistor 39 which is inserted into the meat is, ofcourse, to prevent the temperature of the meat from rising too rapidlyand overshooting its desired temperature established by the timer means84. In other Words, the thermistor 39' serves to maintain the desiredrate of increase of the meat temperature in accordance with the timermeans and if there is a variance from the rate of its temperatureincrease, the immediate oven temperature is changed to restore the meatto its proper program value.

In FIG. 3 curve (c) represents the oven temperature as it varies withtime and curve (d) represents the meat temperature vs. time for theabove described programming system. It will be noted that after theinitial rapid rise of the oven temperature due to the cold meat, thesystem stabilizes itself so that the oven temperature is programmed togradually rise and raise the temperature of the meat at a rate to reachits desired temperature at the end of the time cycle with a minimumamount of energy consumption. The oven is kept just Warm enough tomaintain the steady rise of the meat temperature along its desiredprogram.

While there have been described two systems embodying the teachings ofthis invention, it is recognized many modifications and variations maybe made thereto. It is intended, therefore, to be bound only by thescope of the appended claims.

What is claimed is:

1. An oven heater control system for regulating the cooking temperatureof foods cooked therein comprising thermostat means in said oven,manually operable means for setting said thermostat means to select adesired maximum oven temperature, means operable responsive to thetemperature of said foodfor automatically controlling the setting ofsaid thermostat means to tend to reduce the oven temperature as the foodtemperature increases, and timing means for controlling said thermostatmeans to gradually increase the oven temperature to bring the food to adesired temperature in a desired time period.

2. An oven heater control system for regulating the cooking temperatureof foods cooked therein comprising thermostat means in said oven,manually operable means for setting said thermostat means to select adesired maximum oven temperature, means insertable into said food forproducing a signal varying in a manner relative to the temperature ofsaid food, timer means for varying said signal in an opposite. manner asa function of time, and means operable responsive to said signal forautomatically controlling said thermostat means to gradually increasethe oven temperature to bring the food to a desired temperature in adesired time period.

3. In combination with an electric oven having a heater element and anexhaust vent, a control system for regulating the temperature of saidoven in accordance with the temperature of foods cooked thereincomprising a first bimetal element in said exhaust vent, a first contacton said first bimetal element, a second bimetal element adjacent saidfirst bimetal element, a second contact on said second bimetal in normalengagement with said first contact, means serially connecting saidcontacts with said heater element, said first bimetal element beingdeflectable in a direction to disengage said contacts with a rise inoven temperature, means for manually controlling the pressure *betweensaid contacts to select a desired maximum oven temperature, a secondheater element adjacent said second bimetal element, a negativetemperature coefficient thermistor insertable into said food, apulse-type voltage regulator, a voltage source serially connected withsaid second heater element, voltage'regulator and thermistor, saidsecond bimetal element being defiectable in a direction to lessen thepressure between said contacts with a rise in the heat output of saidsecond heater element, a variable resistance element in series with saidsecond heater element, motor and cam means operable through apredetermined time cycle for operating said variable resistance elementthrough an increasing resistance range, and means for setting said motorand cam means to operate through a desired portion of said cycle.

4. In combination with an electric oven having a heater element, acontrol system for regulating the temperature of said oven in accordancewith the temperature of foods cooked therein comprising a pair ofcontacts in series connection with said oven heating element, meansincluding at least one bimetal element in said oven for maintaining saidcontacts, said bimetal element being defiectable in a direction todisengage said contacts with a rise in temperature, means for manuallycontrolling the pressure between said contacts to select a desiredmaximum oven temperature, a second heater element adjacent said bimetalele- 7 8 ment, 3. negative temperature coefficient thermistor insert-References Cited by the Examiner ahle into said food, and a voltagesource serially connected UNITED STATES PATENTS with said second heaterelement and thermistor, a variable resistance element in series withsaid second heater ele- 7 2,344,820 3/1944 Kearsley 219*492 X rnent;motor and cam means operable through a predeter- 5 2,914,644 11/1959Holtkamp 2195 16 X mined time cycle for operating said variableresistance 2933585 4/1960 Holtkamp e- 219 516 X element through anincreasing resistance range, and means for setting said motor and cammeans to operate through RICHARD WOOD Primary Examiner a desired portionof said cycle. R. F. STAUBLY, Assistant Examiner.

1. AN OVEN HEATER CONTROL SYSTEM FOR REGULATING THE COOKING TEMPERATUREOF FOOD COOKED THEREIN COMPRISING THERMOSTAT MEANS IN SAID OVEN,MANUALLY OPERABLE MEANS FOR SETTING SAID THERMOSTAT MEANS TO SELECT ADESIRED MAXIMUM OVEN TEMPERATURE, MEANS OPERABLE RESPONSIVE TO THETEMPERATURE OF SAID FOOD FOR AUTOMATICALLY CONTROLLING THE SETTING OFSAID THERMOSTAT MEANS TO TEND TO REDUCE THE OVEN TEMPERATURE AS THE FOODTEMPERATURE INCREASES, AND TIMING MEANS FOR CONTROLLING SAID THERMOSTATMEANS TO GRADUALLY INCREASE THE OVEN TEMPERATURE TO BRING THE FOOD TO ADESIRED TEMPERATURE IN A DESIRED TIME PERIOD.